A magnetostrictive torque sensor has been known, for example, of the type having a torque detecting shaft formed on its outer periphery with a magnetically anisotropic portion, so that when a torque is applied to the shaft, any change in the permeability of the magnetically anisotropic portion can be detected by means of a coil provided adjacent the magnetically anisotropic portion. Such a magnetostrictive torque sensor involves a problem that when there arises a change in ambient temperature, a change with time, or the like in its sensor portion including the magnetically anisotropic portion formed on the shaft, an excitation circuit for exciting the coil, and a sensing circuit for processing a signal from the coil, any such change may be a cause of zero error occurrences in torque signal values.
Therefore, for purposes of correcting such zero error, a number of approaches has been proposed including one such that a torque signal value obtainable when no torque is applied to the torque sensing shaft, that is, a zero signal value is calculated and correction is made so that the zero signal value is lower than a predetermined value.
In mechanical areas, such as screw impulse tool, motor-powered screw tool, nut runner, screw machine, and torque tester for testing the performance of those tools, it has been proposed to incorporate a magnetostrictive torque sensor of the above mentioned type or the like for measurement of operating torque in terms of real time.
In screw impulse tools and the like, however, the problem is that since torque load does act in impulse fashion, it is difficult to select only such points of time at which the torque sensing shaft is loaded with no torque, when the tool is in intermittent operation, to calculate a torque signal value at such a point of time, that is, zero signal value.